1. Field of the Invention
The present invention generally relates to a gas detecting apparatus and more particularly to a gas detecting apparatus for detecting gas information such as the sort or species of a particular, gas and concentration thereof or the species of a gas or gases mixed with a known gas at a given or specific ratio as well as concentration of the gas.
2. Description of the Prior Art
Heretofore, semiconductor gas detecting elements made of tin oxide (SnO.sub.2), iron oxide (Fe.sub.2 O.sub.3), zinc oxide (ZnO) or the like which have such a characteristic that the electric conductivity changes in dependence on gas concentration have been widely employed in the gas detecting apparatus because of ease of handling and inexpensiveness. Operation of such semiconductor gas detecting element is based on the principle that when a reducible gas of an electron releasing nature such as hydrogen and hydrocarbon is absorbed on an active surface of an n-type semiconductor such as SnO.sub.2 and ZnO, conductivity of the semiconductor is increased due to an increase in the density of electrons. On the other hand, the conductivity is decreased upon absorption of a gas of an electron pulling nature such as oxygen gas because of decrease in the electron density. On the other hand, in the case of a p-type semiconductor element use is made of the phenomenon that conductivity if decresed upon absorption of the gas of an electron releasing type while absorption of an electron pulling gas involves increasing in the conductivity in reverse to the case of the n-type semiconductor element.
FIG. 1 shows graphically a typical detection characteristic curve 12 representing detection of propane (C.sub.3 H.sub.8) gas by a SnO.sub.2 -detector or sensor element. As will be seen from this figure, the SnO.sub.2 -sensor element can be satisfactorily employed for practical measurements because the conductivity of the element varies as a definite function of concentration of the detected gas. Unfortunately, however, the gas sensor element used almost universely seldom exhibits a selective sensitivity only to a particular sort of gas. Instead, it typically responds with different detection characteristics to various gases such as methane (CH.sub.4), hydrogen (H.sub.2) and ethanol vapor (C.sub.2 H.sub.5 OH.sub.g), respectively, as indicated by curves 11, 13 and 14 in FIG. 1. Besides, the detection output signal produced in response to the presence of combustible gases such as methane, propane and the like is at a lower level than the detection signal produced in response to the presence of organic solvent vapor typified by an ethanol vapor for a given concentration, as will be seen from FIG. 1. Under these circumstances, attempts to realize the selective detection of a specific gas, inter alia the combustible gas, by using the conventional semiconductor gas sensor element have encounted great technical difficulty in dealing with the detection outputs produced in response to the other gases than the desired one.
As an approach to solve the problem mentioned above, there have been proposed a method and an apparatus in which a plurality of gas sensor elements having respective gas detection characteristics differing from one another are used for realizing qualitative and quantitative identifications of a corresponding number of different gases. By way of example, Japanese Patent Application Laid-Open No. 80192/1975 discloses a gas detecting method according to which a sensor element Sab capable of detecting a gas a and another gas b is provided in combination with a sensor element Sb capable of selectively detecting only the gas b for detecting the gas a in case the element capable of detecting selectively only the gas a is unavailable. With this method, the presence or absence of the gas a can be discriminably determined by ascertaining that the element Sb detects the gas b simultaneously with detection of a certain gas by the sensor element Sab. Some inventors of the present application have also proposed a mixed gas detecting apparatus which includes a plurality of gas detecting elements of which gas detection characteristics are previously measured for allowing measurement of the gas information such as species and concentrations of gas components of a gas mixture containing gas components known per se. Reference may be made to U.S. patent application Ser. No. 336,304 filed Dec. 31, 1981, and now abandoned.
However, the method and the apparatus mentioned above as well as those similar to them suffer difficulties mentioned below. Namely, the first mentioned method is considerably limited in respect to the possible combinations of two types of gases which can be measured. This is because the element capable of selectively detecting only one of the gases can not be realized unless the mechanism of gas detection of the gas sensor elements is differentiated for two types of combined gases. Second, the quantitative identification of the gas concentration can not be realized with a signal processing circuit of a simple configuration. This is particularly true in the case where two types of gases simultaneously come into contact with the sensor elements.
The last mentioned proposal has certainly reduced the drawbacks of the former to a significant degree and broadened the range of applications. However, since the number of gas information is inherently less than the number of the incorporated sensors, an increased number of the sensor elements are necessarily required to be provided in order to render more gas information to be available. In this connection, it should be mentioned that the last mentioned system is operative on the basis of characteristic functions which approximate the gas detection characteristics of the individual elements as measured previously, wherein predetermined gas information is derived by processing the gas detection outputs of the individual sensor elements through algebraical procedures. As the consequence, the increased number of the sensor elements will involve surprisingly increased expenditure for executing the algebraical procedures to derive the aimed gas information, which in turn results in a significantly enlarged scale of the signal processing circuit. For example, when a microprocessor is used for the signal processing, storage means of an enormous capacity will be required. Further, accuracy of the arithmetic operation will be degraded due to accumulation of errors occurring in the course of processing. For these reasons, the performances of the gas sensor elements themselves are required to be highly sophisticated in order to dispose of the problems involved by increasing the number of the incorporated sensor elements, although the last mentioned approach may work adequately from the theoritical viewpoint. In this way, there exists a limitation in the number of the sensor elements which can be used in the last mentioned system.